The present invention relates to a method and apparatus for absolutely measuring a diffraction grating spacing, and more particularly to a first method and apparatus which uses a wavelength stable laser of wavelength .lambda., 0th and 1st order diffracted beams and a precision rotary encoder, as well as a second method and apparatus which uses two wave lengths .lambda..sub.1, and .lambda..sub.2, 0th and 1st order diffracted beams and a precision rotary encoder.
In general, there are two conventional techniques for measuring a diffraction grating spacing. The first uses an ordinary laser diffractometer and the second uses a laser interference diffractometer.
1) The method using an ordinary laser diffractometer measures an angle of the diffracted beam whereby the spacing of the diffraction grating is determined by utilizing a diffraction grating equation (1), set out below.
The laser diffractometer is simple in structure, but the measurement error depends on an angle measurement between the incident and diffracted beams. Therefore, in order to obtain a higher measurement resolution capacity, very large experimental equipment is required.
The ordinary laser diffractometer is used mainly for a linewidth measurement of the diffraction grating, or for measuring fine patterns by measuring also the intensity distribution of diffracted beams.
2) The laser interference diffractometer method uses a stable laser with two wavelengths or three wavelengths, which was developed for precisely measuring the diffraction grating spacing by using a standard diffraction grating and a Michelson interferometer.
This method relies upon the fact that spacings of interference patterns of different diffraction orders having mutually different wavelengths are different.
However, as with the conventional ordinary laser diffractometer described above, the length of the measurement arm should be made larger in proportion to the experimental angular resolution. Because there is an eccentricity problem between the diffraction grating and the rotating body, the uncertainty of measurement is decreased when an apparatus of ordinary laboratory size is used.
As with the conventional laser interference diffractometer described above, a stable laser having two or three wavelengths is used along with a Littrow type mounted diffraction grating in one arm of the interferometer in order to measure the diffraction grating spacing. Even though this method can increase the resolution over that of the ordinary laser interferometer, when two wavelengths are used, a measured standard diffraction grating should be used in a reference arm of the interferometer, so that there is a problem that uncertainties in the length standard are indirectly transferred to the measurement.
When three wavelengths are used, the standard diffraction grating may not be used and, because three wavelengths are required, the device becomes complicated.
In the laser interference diffractometer, the interference pattern is measured using a microscope, so that there is a problem of introducing an error into the measurement from the microscope.